Field of the Invention
The present invention relates to a position control system for use in a driving system which transmits driving force of a driving source to a driven member through a power transmission mechanism, a position control method, a program for performing the position control method, and a storage medium which has the program stored thereon. The present invention is suitable for position control for a driven member which has large inertia, such as a development unit or an intermediate transfer drum of an image forming apparatus.
A power transmission mechanism is often provided between a driving source and a driven member. Especially when position control is performed on a driven member (a load) which has relatively large inertia such as a development unit switcher in a multicolor image forming apparatus such as a printer, a power transmission mechanism such as a gear train connects a motor serving as a driving source to a load in many cases in consideration of the efficiency, arrangement and the like of the motor. This is often the case with a DC motor used as the driving source since high efficiency is achieved in driving at a high speed.
The power transmission mechanism always involves a so-called mechanical dead zone (hereinafter referred to as “play”) such as backlash and rattle in a gear train. When a position detector such as a rotary encoder is directly connected to the load, a control system is likely to operate unstably due to the play in the gear train or the like. Also, the encoder needs to deal with pulses at a high frequency to provide a required resolution, thereby causing a higher cost. To avoid these situations, the position detector is often connected to the motor shaft. This is called a semi-closed control system.
To perform position control with high accuracy and little noise, a speed table is often used to control a motor. For example, Japanese Patent Application Laid-Open No. 1982-132797 proposes a control method in which the smoothest possible curve is typically used in the speed table as shown in FIG. 12 in view of noise and the like.
As described above, the power transmission mechanism has play therein. For example, when the power transmission mechanism is used to drive a load which has relatively large inertia, for example a development unit switcher in a multicolor image forming apparatus, a large reduction ratio is set and thus the play is increased.
If such a system is subjected to position control in the aforementioned semi-closed system, the control results may vary greatly depending on an initial state.
For example, when acceleration is performed in accordance with the smooth curve in the speed table as shown in FIG. 12, the inertial loads of a load (a driven member) and a motor are coupled together from the start if the power transmission mechanism has no play in a forward direction in the initial state. When the acceleration curve is appropriately designed, the load and the motor are smoothly accelerated in unison following a command speed as shown by a dotted line in FIG. 13.
On the other hand, if the power transmission mechanism has play in the forward direction in the initial state, only the inertial load of the motor is driven at first. Thus, the position detected by the position detector moves to follow the command speed, but the load does not move.
As the play in the power transmission mechanism is eliminated, the gear train starts elastic deformation. Thereafter, the load cannot be accelerated until the two masses (the motor and the load) start moving in unison as shown in FIG. 14. In FIG. 14, a dotted line shows the motion of the motor, while a dash dotted line shows the motion of the load.
At this point, the position of the load lags behind the position of the motor, and the lag cannot be made up for immediately. As a result, a large overshoot may occur near a target position as shown in FIG. 15.